A polarizing plate has been generally used in a structure in which a protective film is laminated on one surface or both surfaces of a polarizer using an adhesive, the polarizer being formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichroic dye or iodine for durability and water resistance. In this case, as an adhesive used to attach the polarizer to the protective film, a water-based adhesive composed of an aqueous solution of a polyvinyl alcohol-based resin is usually used. However, the water-based adhesive has a problem in that the adhesive strength deteriorates depending on the type of film material, and methods of using a non-water-based adhesive have been proposed in order to overcome the problem. Meanwhile, the non-water-based adhesives may be divided into a radical curable adhesive and a cationic curable adhesive depending on the curing method. The cationic curable adhesive has an advantage in that the adhesive has excellent adhesive strength for films formed of various materials, but has many disadvantages of manufacturing processes due to a slow curing rate and a low degree of curing.
As described above, in the case of a curable adhesive composition used in an adhesive composition, a radical curable composition may be commonly used. The radical curable composition is a very useful material due to convenient workability and fast curability in the manufacturing process of a polarizing plate.
However, the existing radical curable adhesive composition for a polarizing plate is required to have high heat resistance and elastic strength because cracks are generated on a PVA polarizer due to structural characteristics of the polarizing plate when thermal shock is evaluated. In general, in order to increase elastic strength of the adhesive composition, a polymer constituting the composition needs to be crystallized or have a high degree of crosslinking, and the degree of crosslinking of the composition has been increased by using a multifunctional acrylate-based compound such as trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PETA), and dipentaerythritol hexaacrylate (DPHA) as an actual method. However, when the multifunctional acrylate is recklessly used, there is generated a problem in that as a curing shrinkage ratio is increased, adhesive strength with a PVA polarizer significantly deteriorates.
Therefore, in order to solve the problems as described above, there is an urgent need for developing a radical curable adhesive composition having excellent adhesive strength while having high elastic strength.